Existing light emitting diodes (“LEDs”) dies can emit light in the ultraviolet (“UV”), visible or infrared (“IR”) wavelength range. Particularly, UV- and blue-emitting LED dies are commonly utilized for lighting purposes. Such LEDs may also be referred to herein by the color of the light they emit. For example, a blue-emitting LED may be called a blue LED and UV-emitting LED may be called a UV LED. (Similarly, the phosphors described herein may be referred to by the color of light that they emit upon excitation. For example, a yellow-emitting phosphor may be called a yellow phosphor, a green-emitting phosphor may be called a green phosphor, etc.) These LEDs generally have narrow emission spectra which mean that they can not be directly used to produce broad-spectrum light, such as white light with a high color rendering index (“CRT”). Phosphors can be introduced to convert a portion of the light originally emitted by the LED die into light of a different wavelength. The combination of the converted light and the originally emitted light renders a more desirable output light. For example, a white-emitting LED lighting device may contain a blue LED die and a yellow phosphor. The yellow phosphor partially absorbs the blue excitation light and converts it to a broad yellow emission. The combination of transmitted blue light and yellow light renders a white light. The quality of generated white light is determined by the intensity ratio of blue to yellow light and their spectral distributions. The characteristics of the generated white light can be quantitatively measured by color rendering index (“CRT”), correlated color temperature (“CCT”), and chromaticity coordinates (Cx, Cy) on a CIE 1931 color space chromaticity diagram.
Due to the LED die manufacturing process, there is bound to be certain performance variation among the LED dies, for example, in terms of spectral power distribution and emission dominant wavelength. For instance, the emission dominant wavelength of blue LED dies can have a wide distribution and usually ranges from 440 nm to 470 nm. Moreover, the power supplied to drive the LED die and the LED operating temperature may fluctuate during operation and cause an unstable emission spectrum. In contrast, phosphors have absorption peaks at pre-determined wavelengths. For example, a typical YAG:Ce phosphor has an absorption peak near 460 nm and the absorption and conversion decreases when the excitation wavelength deviates from 460 nm. When a single type of phosphor is applied onto LED dies with different emission dominant wavelengths, the output color of the LED device varies due to the difference in the phosphor absorption and conversion at the different excitation wavelengths.